Hydrophilic polyurethane polymers are known chemical entities, the description and preparation of which are set forth, for example, in U.S. Pat. Nos. 3,822,238; 3,975,350; 4,156,066; 4,156,067; 4,255,550; 4,359,558 and 4,451,654 incorporated herein by reference.
Such polymers are obtained by reacting a diisocyanate with a resin having two or more reactive terminal hydrogens and containing various polar sites which appear in the final polymer product and are responsible for its hydrophilic character. Illustrative polar sites include ether groups, carboxylic acid groups, sulfhydryl groups, sulfonium groups, sulfonic groups and quaternary ammonium groups.
Representative resin systems from which the hydrophilic polyurethane polymers can be derived are:
1. An adduct of dihydroxy compounds such as ethylene glycol or propylene glycol with ethylene oxide, propylene oxide, ethylenimine, propylenimine, dioxolane or any mixtures of same;
2. An adduct of trihydroxy compounds such as glycerol or trimethylolpropane with ethylene oxide, propylene oxide, ethylenimine, propylenimine, dioxolane or any mixtures of same;
3. An adduct of tetrahydroxy compounds such as erythritol or pentaerythritol with ethylene oxide, propylene oxide, ethylenimine, propylenimine, dioxolane or any mixtures of same;
4. An adduct of polyhydroxy compounds such as anhydroenneaheptitol, sorbitol, mannitol, hydrolyzed low molecular weight polyvinyl acetate, sucrose or lactose with ethylene oxide, propylene oxide, ethylene imine, propylenimine, dioxolane or any mixtures of same;
5. An adduct of polybasic acids such as trimellitic acid, pyromellitic acid, mellitic acid, pyrophosphoric acid, and low molecular weight polyacrylic and methacrylic acids with ethylene oxide, propylene oxide, ethylenimine, dioxolane or any mixtures of same;
6. An adduct of hydroxy acids such as mal eic acid, citric acid or sugar acids with ethylene oxide, propylene oxide, ethylenimine, dioxolane or mixtures of same. Sugar acids are defined in "Carbohydrate Chemistry," Volume 5, and more specifically, Chapter 17 (a review of literature published during 1971), The Chemical Society, Burlington House, London, Great Britain (1972) and in other sources as well;
7. An adduct of amino compounds, such as ammonia, ethylenediamine, diethylenetriamine, triethylenetetramine with ethylene oxide, propylene oxide, ethylenimine, dioxolane or any mixtures of same;
8. Aminium, iminium or quaternary ammonium salts of 7;
9. A sulfonated polyester resin of maleic acid, itaconic acid, mesaconic acid, fumaric acid and a glycol of 2 to 6 carbon atoms;
10. A polyester of a lower alkyl dialkanolamine and a diacid wherein the diacid is adipic, sebacic, azelaic, maleic, phthalic, fumaric acid or mixtures of same, the amine group being converted to an aminium or quaternary ammonium group;
11. A linear or slightly branched polyamide of an alkylamine and a diacid wherein the amine is diethylenetriamine, triethylenetetramine, tetraethylenepentamine or a polyloweralkylenimine such as ethylenimine or propylenimine. Suitable diacids include maleic, adipic, azelaic, sebacic, phthalic, itaconic acid or any mixture of same. The term "slightly branched" indicates only methyl or ethyl substituents on the polyamide backbone, the ethyl substituent being less than 1%.
12. Aminium, iminium or quaternary ammonium salts of 11;
13. Polysulfhydryl resin having in the backbone sulfonium, sulfoxide, or sulfone groups;
14. Iminium, aminium or quaternary ammonium salts of ethylene or propylenimine adducts of polyhydroxy compounds from categories 1 to 4; and
15. Polyesters of polyethylene oxides with maleic acid, adipic acid, sebacic acid, phthalic acid, azelaic acid, fumaric acid or any mixtures of same.
In general, the resins of classes 1-15 above will have an equivalent weight above 140, preferably above 170, and up to about 2000. In addition, a ratio of carbon atoms to oxygen and/or nitrogen atoms ranging from about 1.2:1 to about 2.8:1 is required. Preferably, the ratio is 1.33:1 to 2.8:1, more preferably 1.33:1 to 2.5:1.
Such polyurethane polymers may vary in their hydrophilicity from polymers that are water soluble to polymers which are water insoluble but which nevertheless will absorb water (e.g., at least 10% by weight) usually accompanied by swelling. Hydrophilic character can be controlled by balancing the type and number of polar sites against the type and size of the inert portion of the polymer molecule following the guidelines given in the cited patents.
Articles produced from the hydrophilic polyurethanes aforesaid are commonly formed by a process known as solvent casting. This is a polymer shaping technique in which a solution of the polymer is applied to a substrate or placed in a mold, and the solvent removed by evaporation. There is obtained a durable plastic article having the configuration of the shaping means. For example, moisture permeable film products such as surgical gloves, condoms, surgical dressings, and the like can be produced by spraying a solution of the polyurethane on to the appropriately contoured support and the coating allowed to dry. Multiple sprayings can be applied when making catheters or other articles requiring greater wall thickness.
Where the polyurethane is water insoluble, at least some of the polymer solution will contain an organic solvent. However, the use of organic solvents poses the risk of fire and toxicity hazards which may necessitate the installation of safety devices and equipment to comply with government standards; pollution controls may also be required. All of these preventative measures add considerably to investment in plant capital and increased operating costs. Furthermore, in solvent casting of hydrophilic polyurethane polymers, it is difficult to remove all traces of solvent. Consequently, the end polymer product tends to retain a small amount of residual solvent. This can be a problem in certain sensitive areas of use, such as where the solvent cast polyurethane article is placed in contact with body tissues which can become irritated by leaching out of the solvent. For instance, the presence of solvent residue in articles for use as catheters, implants, condoms and the like is undesirable.
Clearly there is a need for producing castings of water insoluble hydrophilic polyurethane polymers which do not require the use of an organic solvent.
One possible approach would be to employ the polyurethane in the form of an aqueous emulsion. It is, of course, well known to form polymer emulsions by conducting the polymerization in an aqueous medium. However, this procedure is not practical for preparing aqueous emulsions of a hydrophilic polyurethane since the reactants from which the polymer is synthesized are not compatible with an aqueous medium; the diisocyanate reacts with water and the polyol is soluble in water.